PROJECT SUMMARY Staphylococcus aureus is a versatile pathogen that can colonize the skin and mucous membranes of mammals without causing symptoms, but it can suddenly produce a life-threatening infection. The mechanism by which this transition occurs is not known. Staphylococcus aureus accounts for 300,000 hospitalizations and 11,000 deaths annually, resulting in direct heath care costs of more than $4.5 billion in the United States. The successful pathogenicity of S. aureus may be due to its ability to adapt to different metabolic environments, and it is possible that differences in the metabolic environment in a particular host may cause a transition of these bacteria from a non-pathogenic status to a highly pathogenic status. Regulation of the expression of virulence factors is important to S. aureus for survival, growth, and pathogenicity. The most prominent virulence factors of S. aureus are staphylococcal exotoxins (cytotoxins and enterotoxins) that disarm the host immune system by causing lysis of leukocytes or aberrant activation of immune system cells, leading to shock. Another important virulence factor is alteration of bacterial cell wall structures that confers evasion from host immune surveillance and resistance to antibiotics. Metabolism is an integral process by which nutrients are assimilated into energy and biomass. Considerable evidence indicates that metabolic adaptation to local nutrient availability can greatly affect the pathogenicity of S. aureus by altering expression of key virulence factors. However, very little is known about the role of particular carbon sources (carbohydrates) in the pathogenicity of S. aureus or the underlying regulatory networks linking metabolism and pathogenicity. Our long-term goal is to determine the molecular mechanisms by which metabolic adaptation by S. aureus increases virulence and antibiotic resistance. Our central hypothesis is that the expression of specific virulence factors is linked to the metabolism of specific carbohydrates and that this process is regulated by carbon catabolite repression (CCR) pathways. This will be investigated by pursuing the following specific aims: 1) Determine the effect of particular carbohydrates on alteration of S. aureus pathogenicity and antibiotic resistance, and 2) Determine the role of carbon catabolite repression in metabolic adaptation of S. aureus. We expect this study to lead to identification of specific carbohydrate-driven changes in the molecular architecture of the cell wall of S. aureus which decreases recognition by innate immune cells and increases resistance to antibiotic. It is also expected that this study will reveal key metabolic intermediates regulating in metabolic adaptation which may be exploited pharmaceutically to develop novel therapies against S. aureus.